1. Field of the Invention
The present invention relates to an optical amplifier apparatus and a controlling method thereof, and an optical transmission system using the optical amplifier apparatus, and it relates, in particular to the optical amplifier apparatus and the optical transmission system being suitable to be applied to a wavelength multiplexing optical transmission system.
2. Description of Related Art
In recent years, accompanying with requirement of a low cost optical system, an optical transmission system of so-called a wavelength multiplexing optical transmission system has been studied, in which a plurality of optical signals different in wavelength thereof are multiplexed to be transferred through a single optical transmission fiber.
On the other hand, an optical amplifier apparatus, since it has a wide range in wavelength of optical signal to be amplified therewith and it has an ability of amplifying with low noise, is suitable for use as an amplifier apparatus in the wavelength multiplexing optical transmission system. Optical fiber added with a rare-earth material or metal therein or a semiconductor amplifier, which can construct the optical amplifier apparatus, has a dependency on the wavelength in gain thereof, therefore, a difference is occurred in an optical output or in the gain, for each wavelength, after amplification therewith.
The above-mentioned difference in the wavelength is added up or integrated, in particular in multistage in-line amplification with the optical amplifier apparatuses, thereby increasing the difference in optical power for the each wavelength. As a result of this, a maximum transmission distance in a total system is restricted by deterioration in a S/N ratio of the optical wavelength having the lowest power among the multiplexed wavelengths. Accordingly, it is very important to provide an optical amplifier apparatus having an ensured characteristic of flatness, i.e., no difference in the optical output for every wavelength, in the gain thereof.
Therefore, as a conventional method, there has been already known a method, xe2x80x9cFlattening of characteristic in collective amplification of multi-wavelengths with an optical fiber amplifier using a control of amplification factor of fiberxe2x80x9d, in Institute of Electronics, Information and Communication Engineers of Japan, Technical Paper OCS94-66, OPE94-88(1944-11), for example.
In the conventional method mentioned in the above, a characteristic curve of wavelengthxe2x80x94optical power which is complex and variable in the shape thereof with respect to changes in an input power is made a constant or flat under a predetermined condition, thereby the characteristic curve in the gain is compensated under the predetermined condition.
Namely, an optical signal which is multiplexed with four waves in wavelengths of xe2x88x9211 dBm is inputted into an optical amplifier, and an optical output as a total of the amplified optical signal is monitored, wherein a fiber gain controller (it is called as xe2x80x9cAFGCxe2x80x9d hereinafter) for controlling fiber gain is used so as to make a level of that output a constant value. In this manner, the fiber gain can be controlled at the constant value of 12 dB, thereby minimizing the difference for each wavelength.
Or, by use of an automatic power controller (called as xe2x80x9cAPCxe2x80x9d hereinafter) with an optical attenuator, optical loss is adjusted while maintaining the fiber gain at the constant value of 12 dB, thereby inhibiting the changes in spectrum of the fiber gain if the amplification factor of the in-line amplifier is changed.
In an actual application of the optical amplifier apparatus to the optical transmission system, it is conceivable that a transmission span length is not always constant. One example of such the application is explained with referring to FIG. 9.
FIGS. 9(a)shows a block diagram in a case when the optical amplifier apparatus is applied to the optical transmission system in which the transmission span is not constant. In the figure, a reference numeral 1 denotes an optical receiver, 2 an optical amplifier apparatus, 3 an optical multiplexer, 4a, 4b, 4c optical transmitters. The distance from the optical transmitter 4a to the optical multiplexer 3, that from the optical transmitter 4b to the optical multiplexer 3, and that from the optical transmitter 4c to the optical multiplexer 3 are different from one another.
As shown in FIG. 9(a), wavelength multiplexed transmission optical signals from the optical transmitter 4a, the optical transmitter 4b and the optical transmitter 4c, passing through the optical multiplexer 3, are amplified by the optical amplifier apparatus 2, thereafter they are distributed to the optical receiver 1.
As shown in FIG. 9(b), time bands, during which the wavelength multiplexed transmission optical signals from those optical transmitters 4a, 4b and 4c are distributed to the optical receiver 1, are pre-assigned, respectively. Namely, this shows a transmission method of so-called a time division multiplex (hereinafter, called as only xe2x80x9cTDMxe2x80x9d), in which the optical receiver 1 receives the optical signals in a time sequence from the predetermined one of the optical transmitters.
In the method mentioned in the above, the transmission distance from the optical transmitter to the optical amplifier apparatus is not constant, therefore, a level of the optical input at the optical amplifier is low, during the time band when the wavelength multiplexed optical signals from the optical transmitter at long transmission distance are distributed. On the contrary, during the time band when the wavelength multiplexed optical signals from the optical transmitter at the short transmission distance are distributed, the level of the optical input at the optical amplifier is high.
There is a drawback that it is necessary to apply an optical amplifier apparatus having a wide dynamic range for input, in order to achieve equal optical amplification for all of such the optical input signals.
Further, a system of such construction is also conceivable that a plurality of optical amplifiers are located at positions where they are not necessarily constant in the transmission span. If the transmission span differs, the span loss also differs, then the input level of the optical signals at the optical amplifier differs depending on the location where it is positioned. Therefore, there is a drawback that it is desired to apply an optical amplifier which possesses the input dynamic range being able to cope with any length of the transmission span, in order to construct a transmission system of high reliability with ease and with certainty.
However, the conventional art mentioned in the above has studied only the case where the input level is fixed at xe2x88x9211 dBm. And, in the method mentioned in the above conventional art, only if the dynamic range from xe2x88x9230 dBm to 0 dBm can be secured with respect to the input, for example, then the fiber gain is made constant at 12 dBm.
Therefore, the output level of the fiber changes from xe2x88x9218 dBm up to +12 dBm depending on the level of the signals. At this time, when the optical output is controlled at constant all over the input dynamic range by use of the APC, the optical output mentioned above must be below xe2x88x9218 dBm, i.e., about one-hundredth ({fraction (1/100)}) of the ordinal optical transmission power, thereby causing a problem in practical use.
For dissolving such the problem as mentioned in above, it becomes a necessary object to increase the gain above 12 dB, however, in general, the optical fiber which is added with the rare-earth metal therein has such a problem, that the higher in the gain thereof, the more difficult to realize the flatness in wavelength. In the conventional method mentioned in the above, though it studied only the case of the small gain amplification of 12 dB, but it fails to study a method or countermeasure for ensuring a gain over 30 dB and for realizing the flatness in those wavelengths.
Furthermore, if the gain of the optical fiber is increased, a saturation is caused in the gain due to the characteristics of the optical fiber added with the rare-earth metal when the input signal becomes large, as far as the gain is controlled at constant as taught by the conventional method. Therefore, it is principally impossible for it to ensure the gain over 30 dB within the range of all the above-mentioned input dynamic range, after all, there is a problem that it is difficult to enlarge the input dynamic range.
Namely, the conventional art mentioned in the above is that which discloses only the characteristic within the extremely restricted condition, however, there are many problems to be solved upon application thereof to an actual system.
Further, the wavelengths, 1,548 nm, 1,551 nm, 1,554 nm and 1,557 nm, for examples, which are applied into the conventional method, are those wavelengths, within a region of which the flatness is realized relatively. However, in order to further enlarge the applicable region of wavelength over those, there is caused another problem that it is necessary to further enlarge the flattened region of the fiber itself.
Further, in totality of the optical amplifier apparatus, the optical attenuation, which comes to be the loss of the gain, deteriorates efficiency in amplification by the optical amplifier apparatus as a whole, or comes to be a factor of lessening noise index thereof. In the conventional example, though the optical attenuator is used, however, ill effect affected by the optical attenuation on the optical amplifier apparatus and a countermeasure therefor are not studied at all.
Moreover, as an general characteristic of the optical amplifier apparatus, it has already been known that a degree in the flatness for wavelengths is varied depending on a change in the number of the multiplexing of wavelengths. In the conventional art mentioned in the above, the wavelength flatness is studied when it is restricted to the multiplexing of four (4) wavelengths, however, there is a problem that no countermeasure for a case that the number of the wavelength multiplexing is changed has been studied.
An object is, in accordance with the present invention, for dissolving the drawbacks in the conventional art mentioned in the above, to provide an optical amplifier apparatus and a control method thereof, and an optical transmission system using the optical amplifier apparatus, having the wide input dynamic range, realizing the flatness in wavelengths of a large number of the optical signals, easily, with only a small number of active devices, suppressing deterioration in the amplification efficiency and the noise index corresponding to change in each wavelength and the number of the wavelength multiplexing, and realizing the wavelength flatness, automatically.
For achieving the above object, in accordance with the present invention, there is provided an optical amplifier apparatus, for amplifying inputted multiplexed optical signals, comprising:
optical adjusting means for adjusting the inputted multiplexed optical signals in optical power thereof with a certain characteristic of gain ;
optical amplifying means for amplifying the optical power of the adjusted multiplexed optical signals with a characteristic of gain which is reversed to that of said adjusting means;
optical splitting means for splitting a part of the amplified multiplexed optical signals as an output optical signal and for splitting another part thereof as a detection optical signal;
optical signal detecting means for inputting said detection optical signal which is splitted; and
a controller apparatus for controlling said adjusting means and said optical amplifying means upon an output from said optical signal detecting means, wherein an output gain characteristic in the optical power of said optical amplifying means is flatten at a predetermined value.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said adjusting means comprises: an splitting portion for splitting the inputted multiplexed optical signals into a plurality of optical signals of wavelength bands; a pumping light source having an optical splitting element for splitting an emitted pumping light; a plurality of multiplexers for multiplexing said plurality of split optical signals and said split pumping light; a plurality of optical fibers doped with rare-earth material, being different in amplification factors thereof; a plurality of optical band-pass filters for by-passing only the optical signal of a desired wavelength band among said multiplexed optical signals which are amplified; and an optical multiplexer for multiplexing the plurality of the optical signals of the wavelength bands by-passing said filers.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said optical amplifying means comprises: a pumping light source for emitting a pumping light; an optical multiplexer for multiplexing said inputted multiplexed optical signals and the pumping light from said pumping light source; and an optical fiber doped with rare-earth material for amplifying said multiplexed optical signals.
Further, in accordance with the present invention, there is also provided an optical amplifier apparatus as defined in the above, wherein said optical signal detecting means comprises: an optical splitting portion for splitting the split optical signals into two paths; a first optical signal detector connected to said dividing portion; and a second optical signal detector connected to said dividing portion through an optical band-pass filter for by-passing only the optical signal of a specific wavelength.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said controller apparatus comprises: a first controlling means for controlling said optical amplifier means upon a first monitor signal from said first optical detector; and a second controlling means for controlling said adjusting means upon the first monitor signal and a second monitor signal from said second optical detector.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said optical fiber doped with the rare-earth material is an Erbium doped optical fiber.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said first optical signal detector detects a total optical power of the optical signals of the plural wavelength bands to provide the first monitor signal, said second optical signal detector detects the optical power of the optical signal of the specific wavelength through the optical band-pass filter to provide the second monitor signal.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said first controlling means compares said first monitor signal to a reference voltage to control the pumping light source of said optical amplifier means with a comparison signal thereof, thereby adjusting a level of the wavelength characteristic in the gain of said optical amplifying means.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein said second controlling means compares an average optical power of said plurality of the optical signals of wavelength band, which is obtained by dividing said first monitor signal and an optical power of the optical signal of the specific wavelength of said second monitor signal, thereby controlling the pumping light source of said adjusting means with the difference signal so as to adjust a level and an inclination of the characteristic in the gain of said optical amplifying means being reversed to the wavelength characteristic which is possessed by said adjusting means.
Further, in accordance with the present invention, there is provided an optical amplifier apparatus as defined in the above, wherein the optical band-pass filter of said adjusting means is positioned at a high gain end or a low gain end of the multiplexed optical signal.
Moreover, also for accomplishing the above object, in accordance with the present invention, there is provided a control method of an optical amplifier apparatus for compensating gain characteristic of an optical amplifying medium, wherein causing a gain characteristic being reversed to a characteristic in gain of said optical amplifying medium, thereby flattening the gain characteristic possessed by said optical amplifying medium at a predetermined value.
Further, in accordance with the present invention, there is provided a control method of an optical amplifier apparatus as defined in the above, wherein said reversed gain characteristic is changeable.
Further, in accordance with the present invention, there is provided a control method of an optical amplifier apparatus as defined in the above, further comprising steps of:
detecting a level of an optical output from the optical amplifying medium so as to control the gain characteristic thereof; and
detecting flatness in the gain of the optical output so as to control said reversed gain characteristic.
Further, in accordance with the present invention, there is provided a control method of an optical amplifier apparatus as defined in the above, further comprising steps of:
detecting total optical power of a plurality of optical signals of an wavelength band, detecting optical power of only an optical signal of a specific wavelength by using an optical band-pass filter, and adjusting the gain characteristic value of the optical amplifying medium upon a comparison signal between said detected total optical power and a reference voltage; and
adjusting a level and an inclination of said reversed gain characteristic by a comparison signal between the averaged optical power of the plurality of the optical signals of the wavelength band, which is obtained from said total optical power and the optical power of said optical signal of the specific wavelength.
In addition to the above, also for accomplish the above-mentioned object, in accordance with the present invention, there is further provided a multiplexed optical signal transmission system having a optical transmitter apparatus, an optical amplifier apparatus, an optical receiver apparatus and multiplex optical fibers connecting among those, wherein the optical amplification apparatus as defined in one of the claims is used therein, said optical amplifier apparatus is positioned at an after-stage of said multiplex optical fibers.